In the formation of high density integrated circuits, there is a need to isolate the components from one another in order to prevent undesired interactions. One means of achieving isolation, in the art, is to etch a series or pattern of isolation trenches in the semiconductor substrate, passivate them with oxide and nitride and then fill them with a dielectric material. When a dielectric material such as polyimide is used, the typical processing consists of finishing all high-temperature processing including formation of emitter contacts and emitter diffusion prior to the formation of the isolation trenches. In filling the trenches, the dielectric material is deposited over the entire patterned substrate so as to overfill the trenches. The dielectric material is then etched back to re-expose the substrate and contacts, leaving the trenches "full". In order, however, to guarantee exposure of the contacts, which are located in recesses in the substrate surface, the dielectric material is generally over-etched below the recess level, thus leaving a significant step at the trench peripheries. This topographical non-planarity has been found to create discontinuities in the subsequently formed metallization line patterns.
A further well-known difficulty with regard to the isolation trenches is in the continuous nitride passivation of the substrate, especially at the trench peripheries. In a typical process, the top substrate surface is passivated with the nitride before trench etching; the trench sidewalls are then passivated with a subsequent separate nitride deposition. As such, at the edge, the protection of the underlying commonly used oxide insulator is relatively poor. The result of the poor nitride passivation can include: silicon-tometal shorts near the top edge of the trenches as a consequence of accidental removal of the oxide insulator in a cleaning or etching step that follows; or contamination of the insulating layer with, for example, sodium from a subsequently deposited quartz layer.